"smart" expression jack

[duffrecords]

I have three pedals with expression inputs, so I built a signal routing box with a rotary switch so I can select which pedal I'm controlling with my expression pedal.  It even has an on/off switch so I can choose not to control any pedals.  So far, so good.

One exception is my Rainbow Machine.  The Rainbow Machine's expression in operates the pitch control and, since it uses a normalled switching jack, inserting a cable physically breaks the connection to the pitch knob.  When I select one of the other pedals with my routing box (or turn it off), the cable is still connected to the Rainbow Machine's expression in, so the pitch knob remains bypassed.  I want the option to set that knob manually without having to unplug the expression cable.  My whole pedalboard is set up so that I never have to plug/unplug a cable; I just use switches.

I was thinking one way to do this, since the pedal applies a voltage across the pot in the expression pedal, is to use PNP transistors inside the pedal as toggle switches to open or close the normalled contacts on the expression out.  That way, if no cable is present, the contacts are physically closed.  If a cable is inserted and the Rainbow Machine is selected on the routing box, current will flow through the return part of the expression cable (I forget if that's the tip or ring) and it will forward-bias the PNP and open the contacts, defeating the pitch knob.  If a cable is connected and no current flows through the return (i.e. the Rainbow Machine is not selected on the routing box), the PNP shuts off and the contacts are closed again, enabling the pitch knob.

Am I on the right track or is there a simpler way to do it?  This could be done with a mechanical switch but I'd prefer to make it "smart" enough to sense the connection to the expression pedal.

[samhay]

Does the rainbow machine use a stero or switched jack for the expression input?

[duffrecords]

Both.  It's a stereo TRS jack (send, receive, and ground) and it's one of those closed-circuit jacks where there are two solder lugs for each contact.  When you insert a plug, it opens the connection between the two lugs.  This is how it disables the pitch knob.

[robthequiet]

Edit: It could be as simple as shorting the middle leg of the jack to the sleeve. Where I hesitate is not knowing if there is more to the circuit than a simple connection break. Logically, hard-wiring one solution will void the other, unless a simple short works, in which case inserting a microswitch to override the normalized break might be a way to go. One more switch to manage, though.

[duffrecords]

My guess is the circuit works like the picture below.  I could try shorting those contacts but, yeah, I definitely don't want to add another toggle switch.  That gets hard to keep track of in a live scenario.

[duffrecords]

I think that simply shorting the contacts would have the effect of connecting the expression pedal's pot and the pitch knob in parallel, which would reduce the overall resistance.  I'm not sure what would happen in that case.  I do know that opening and closing the contacts that lead to the pitch knob disable and enable it, respectively.

There's a transistor guide on Sparkfun that gave me (and several other people, it seems) the wrong idea about PNP transistors, that they function like a normally-closed switch until current is applied to the base.  What I need is a NOT gate that breaks the connection to the pitch knob when current flows through the expression pedal and makes the connection in the absence of current.  The potential problem I see with this approach is the voltage drop across the base resistor might be enough to skew the range of the expression pedal's operation.

[robthequiet]

Good points. You could build a switching circuit with some isolation, if you know where to insert it, with the not gate for buffering. Without a schematic, though, it's all theoretical. I don't know how inclined you are to doing surgery on your Rainbow Machine. Here is an idea: When you have your selector switch off the Rainbow Machine, can it connect instead to an independent knob on the expression pedal that adjusts the RM pitch? That way you could still have the ability to select another device for expression pedal control and use the extra knob as a dedicated preset for the Rainbow Machine? Then when you select the RM, it switches away from the extra knob and connects to the expression pedal pot? Does this make sense? This way you still leave the Pitch control on the RM disabled but you can still set the level independently.

[duffrecords]

Thanks, that's a great idea!  After all, the Rainbow Machine is full of surface mount circuitry that I'd rather not tamper with.  The rotary knob in my selector box is a 4P3T, so it would probably work if I wired it something like this:

[robthequiet]

I think that will work. Test it and let us know how it goes. Lucky that you have that extra pole on the switch.

[samhay]

First thing first - if you plug a stereo (TRS) cable into the expression jack, does this disable the pitch control?

[duffrecords]

That's exactly what happens--plugging a stereo TRS cable into the expression jack disables the pitch knob so the expression pedal can control that function instead. It does so by opening the contacts on the closed-circuit switching jack, which I assume lead to the potentiometer under the pitch knob.

[samhay]

OK - it's odd that it needs a stereo cable as you should only need the expression voltage and ground - i.e. a mono cable with ground on the sleeve and the tip switched between the pitch pot wiper and the expression pedal.

Edit - ignore previous bit - makes perfect sense for a passive expression pedal if it is running as a potentiometer.

If you don't want to mod the pedal (which is understandable), then you should be able to put a remote pitch control knob on your switching box so that when the expression control is not selected for the rainbow machine, it switches in the new pot in instead, but I would have thought this is likely to be set up as a voltage divider, not a variable resistor.

[anotherjim]

I think Sam's suggestion is very sensible. A pot on the routing box that duplicates the expression pedal when the pedal is routed elsewhere.

[duffrecords]

You're right. It would be a voltage divider. I was just in a rush and forgot to put the ground symbols on my schematic.

[duffrecords]

I haven't rewired my selector pedal yet but here's the basic schematic for reference:



I'm considering making it a precision potentiometer like this because it's near impossible to dial the Rainbow Machine's pitch knob to unity.  It's just to the right of 12:00 with no detent and you need a delicate touch and patience to find it.  The only problem is I probably only have room for one more pot inside this box, if I'm lucky.